The composition of liquefied natural gas (LNG) changes in the context of storage in a tank, depending on the storage duration and the storage temperature. It is furthermore known that the composition of liquefied natural gas differs in country-specific fashion, and for example that the proportion of methane and also of residual constituents is different. The power generated by an internal combustion engine operated with liquefied natural gas, as well as the composition of the exhaust gases emitted by it, depend on the composition of the liquefied natural gas. It is therefore necessary to adapt the combustion parameters of an internal combustion engine operated with liquefied natural gas to the currently available composition.
It is known from the existing art to store the combustion parameters of an internal combustion engine operated with liquefied natural gas upon shutoff of the internal combustion engine, and to use them at the next startup as initial parameters for controlling combustion. In this context, data regarding the composition of the gas/air mixture are sensed from lambda probes and stored, and data regarding mixture adaptation and natural gas quality are also stored.
German Patent No. 60001644 T2 describes a method for identifying the proportion of one of the components, or the ratio of two of the components, of a fluid mixture of liquid natural gas, having the method steps of measuring the thermal conductivity of the liquid natural gas at a first temperature, measuring the thermal conductivity of the liquid natural gas at a second temperature, and deducing the proportion of the one of the components, or the ratio of two of the components, of the liquid natural gas from the two thermal conductivity measurements.
The document furthermore describes an engine management system for an engine operated with LNG, having an apparatus as recited in one of the claims of German Patent No. 60001644 T2 for identifying the proportion of one of the components, or the ratio of two of the components, of LNG fuel, and having means for modifying one or more parameters in accordance with the ascertained proportion of one of the components, or the ratio of two of the components, of the LNG fuel. This existing art is disadvantageous in that upon startup of the internal combustion engine, the current composition of the liquefied natural gas must first be identified.